The present invention relates to water-based ink-jet ink compositions that are miniemulsions, i.e., an aqueous vehicle having emulsified oil particles with dissolved dye molecules, the oil particles having a diameter below 1 xcexcm.
The use of ink-jet printing systems has grown dramatically in recent years. This growth may be attributed to substantial improvements in print resolution and overall print quality coupled with appreciable reduction in cost. Today""s ink-jet printers offer acceptable print quality for many commercial, business, and household applications at costs fully an order of magnitude lower than comparable products available just a few years ago. Notwithstanding their recent success, intensive research and development efforts continue toward improving ink-jet print quality, while further lowering cost to the consumer.
An ink-jet image is formed when a precise pattern of dots is ejected from a drop-generating device known as a xe2x80x9cprintheadxe2x80x9d onto a printing medium. The typical ink-jet printhead has an array of precisely formed nozzles located on a nozzle plate and attached to an ink-jet printhead substrate. The substrate incorporates an array of firing chambers that receive liquid ink (colorants dissolved or dispersed in a solvent) through fluid communication with one or more ink reservoirs. Each chamber has a thin-film resistor, known as a xe2x80x9cfiring resistor,xe2x80x9d located opposite the nozzle so ink can collect between the firing resistor and the nozzle. In particular, each resistor element, which is typically a pad of a resistive material, measures about 35 xcexcmxc3x9735 xcexcm. The printhead is held and protected by an outer packaging referred to as a print cartridge, i.e., ink-jet pen.
Upon energizing of a particular resistor element, a droplet of ink is expelled through the nozzle toward the print medium, whether paper, transparent film or the like. The firing of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements, thereby forming alphanumeric and other characters on the print medium.
Inks normally used in ink-jet recording are commonly composed of water-soluble organic solvents (humectants, etc.), surfactants, and colorants in a predominantly aqueous fluid. When a recording is made on xe2x80x9cplain paperxe2x80x9d, the deposited colorants retain some mobility, which can be manifest in poor bleed, edge acuity, feathering, and inferior optical density/chroma (due to penetration of the paper). These features adversely impact text and image quality.
There is a considerable demand for better permanence of ink-jet images. The following permanence attributes are important, but hard to achieve:
1. Waterfastness, that is, stability of the image to water drip;
2. Highlighter smearfastness, that is, stability to highlighter smear;
3. Stability to wet and dry finger smudge.
Waterfastness of inkjet inks can be controlled in different ways and can be achieved both for dye- and pigment-based inks, as exemplified, e.g. by Canon 8500 inkjet printer output in all the colors, and HP890C output in black. However, smearfastness with both wet and dry smudge is more difficult to achieve. One of the approaches is to add a polymeric binder to the inks, as described, e.g. in the international patent applications WO 99/50364 by Yeates et al. and WO 99/50365 by Yeates et al. In these applications, it is suggested to use a polymeric polyurethane, or a polyacrylate in combination with an apolar solvent as an additive to dye-based ink in order to improve the smudgefastness. The polymer loading into the ink formulations is rather high, xcx9c10% and potentially can cause the pen reliability problems Alternatively, adding a UV-curable monomer/oligomer to the inks has been proposed, (see H. Noguchi, UV-Curable Aqueous Ink Jet Ink, ISandT""s NIP 14:1998 International Conference on Digital Printing Technologies, p. 107, or U.S. Pat. No. 5,623,001 by M. Figov xe2x80x9cInk Compositions and a Method for making Samexe2x80x9d). The problem here is the objectionable smell and toxicity, potential instability of the monomers in the inks, poor curing of water-based systems, and dye fade by UV.
An approach to improve the attributes described above are various xe2x80x98emulsionxe2x80x99 techniques. Emulsions represent plurality of droplets of one liquid in another. Emulsions can have a different type, e.g., oil-in-water(O/W) or water-in-oil (W/O). In addition to this, emulsions are divided on microemulsions and macroemulsions. Microemulsions are thermodynamically known as xe2x80x9cswollen micellesxe2x80x9d, which means that they represent surfactant micelles with the oil solubilized inside. Typical microemulsion particle size is 5-50 nm. Microemulsions are normally transparent or slightly bluish because of the very small particle size. One the other hand, macroemulsions are not in thermodynamic equilibrium and do not form spontaneously, that is, they require mechanical agitation for preparation. Their particle size is larger, 50 nm and up. One of the common examples of macroemulsions is milk.
The use of microemulsions in ink-jet is known. Thus, in the patent of Wickramanayake et al., (U.S. Pat. No. 5,226,957) entitled xe2x80x9cSolubilization of Water-insoluble Dyes via Microemulsions for Bleedless, Non-Threading, High Print Quality Inks for Thermal Ink-jet Printersxe2x80x9d, it is suggested to incorporate an oil-soluble dye into a microemulsion, which contained solvents, surfactants, co-surfactants, and water. In another patent of Wickramanayake (U.S. Pat. No. 5,565,022, xe2x80x9cFast-Drying, Bleed-Free Ink-Jet Ink Compositionsxe2x80x9d), it is suggested to incorporate a dye into a mixture of an organic solvent, surfactant, and water, so that the dye and water-insoluble solvent are solubilized by the surfactant. In U.S. Pat. No. 5,643,357 (Brenton et al., xe2x80x9cLiquid Crystalline Ink Compositionsxe2x80x9d), an ink composition is suggested, that contains a surfactant, and oil-or-alcohol-soluble dye, and water. The formulation undergoes a phase transition upon heating from a microemulsion phase to a lamellar phase, which helps to stratify the inks to the surface of the paper, when the printing substrate is heated. In the U.S. Pat. No. 5,492,559 (Oliver et al., xe2x80x9cLiquid Crystalline Microemulsion Ink Compositionsxe2x80x9d), an ink formulation is described containing an aqueous phase, oil phase, and surfactant, and an oil-soluble dye, such that the system undergoes a microemulsion-liquid crystalline state transition with decreasing temperature. The idea is (with some variations) to solubilize water insoluble xe2x80x9csolventxe2x80x9d dyes in micelles, and thereby, produce water and smudgefast images.
The use of miniemulsions for inkjet printing has been discussed previously by several authors. In U.S. Pat. No. 5,345,254 by Wong et al., the authors describe the use of an emulsion or suspension of an organic phase in a water phase, so that the organic phase includes at least one oil and wax. In U.S. Pat. Nos. 5,837,753 and 5,746,815 authored by Caputo, stable oil-in-water ink emulsions containing dissolved solvent dyes are described. The systems contain an organic solvent of citrus origin, and a polymeric binder. Spinelli in U.S. Pat. No. 5,772,741 describes an ink formula, containing a non-aqueous phase and a pigment dispersed in water.
However, the need remains for better inkjet ink emulsion formulations. Thus, all the microemulsion formulations fall short of attaining a good print quality because of the excessive ink penetration into the paper and wicking along the fibers. As for the miniemulsions concerned, the need remains for better particle size control and emulsion stability. Also, the need remains in solvents that have less smell, lower toxicity, and higher solubilizing capacity towards the solvent dyes. Finally, the need remains for a solvent system that combines all these attributes together, which is not a trivial task. The problem here is related to the fact that in order to achieve a significant solvent dye solubility in the oil, the oil must be somewhat polar, to match the polarity of the dye chromophores. However, polar oils tend to produce unstable miniemulsions in water because of Ostwald ripening (Kabalnov, A. S. and Shchukin, E. D., xe2x80x9cOstwald Ripening Theory: Application to Fluorocarbon Emulsion Stability,xe2x80x9d Adv. Colloid Interface Sci. 38:69-97 (1992)). The present invention provides the solvent system, previously unknown, that combines these attributes together.
The present invention relates to an aqueous ink-jet ink composition comprising:
(a) at least one oil-soluble dye;
(b) at least one oil/solvent system; and
(c) an aqueous vehicle,
wherein particles of the oil-soluble dye are dissolved in low-polarity oil particles having a particle size less than 1 xcexcm, the particles forming a miniemulsion in the water.
The present invention further relates to a method of making an aqueous ink-jet ink composition comprising the steps of:
(a) dissolving an oil-soluble dye in a oil/solvent system to form a dye-containing oil;
(b) adding the dye-containing oil to an aqueous vehicle;
(c) mixing the dye-containing oil into the aqueous vehicle to form a mixture; and
(d) emulsifying the mixture to form an oil-in-water emulsion having dye-containing oil particles no larger than 1xcexcm in diameter.